There are a number of limitations to this study. Our heroin and cocaine dependent populations were grouped together in the association analyses to make an ‘addicted’ cohort. It is possible that the variants which were not associated with drug addiction still affect risk for heroin or cocaine addiction. We were not able to split our cohort for heroin or cocaine specific analyses while maintaining sufficient statistical power. However, there is a substantial co-morbidity between heroin and cocaine addiction; 50% of intravenous cocaine users report using heroin on a regular basis [24] and 92% of heroin users also use cocaine [18]. Splitting our sample into heroin and cocaine dependents would not adequately assess the contribution of these variants to ‘pure’ cocaine or heroin addiction as such individuals are rare in the population.
